There have been power semiconductor devices including a silicon carbide (SiC) substrate, such as a Schottky diode, a pn diode, a metal oxide semiconductor field effect transistor (MOSFET), and an insulated gate bipolar transistor (IGBT). SiC has dielectric breakdown electric field strength greater than that of Si, so that the semiconductor devices including the SiC substrate can also be used in an extra-high-voltage region (greater than or equal to 10 kV) to which Si is not applicable.
In the extra-high-voltage region, a drift layer having a low concentration and a great thickness is used to keep withstand pressure. Consequently, when a semiconductor device is used as a unipolar device, a drift resistance and an ON resistance are likely to increase. Thus, a bipolar device is often used to reduce the ON resistance. Examples of a bipolar device include a pn diode and an IGBT, for example. In the bipolar device, carriers including both electrons and positive holes contribute to conduction, so that a drift layer at a low concentration seemingly functions as if the drift layer is doped at a high concentration (conductivity modulation effects), and the ON resistance is significantly reduced.
To discuss performance of the bipolar device, a time constant (lifetime of carriers) in which excess carriers disappear due to recombination is an important index. The longer lifetime further enhances the conductivity modulation effects in the bipolar device. As a result, the ON resistance can be reduced. On the other hand, if the lifetime is too long, the accumulation of the carriers reduces switching characteristics of the bipolar device, which increases a switching loss. Therefore, the lifetime needs to be properly controlled according to the intended purpose of the device.
In indirect transition semiconductors such as Si and SiC that are normally used as materials for power devices, the lifetime increases because a recombination speed of an electron and a positive hole between bands is slow. On the other hand, however, the band gap have an energy level (defect level) when the semiconductor materials have impurities, intrinsic defects, and crystal defects such as dislocation, and stacking faults. The electron and the positive hole recombine with each other through the defect level in some cases, and the defect is referred to as a recombination center. With a plurality of recombination centers, the lifetime of the semiconductor material is represented by an inverse of a sum of an inverse of a lifetime in each recombination process. Thus, a process in which a lifetime is the shortest among the plurality of recombination processes limits the rate of the lifetime of the semiconductor material.
Therefore, in an indirect transition semiconductor, a lifetime is not a transition between intrinsic band gaps of a semiconductor material, and a lifetime is determined by a recombination center. Particularly, the recombination center that is a main cause of reducing a lifetime is referred to as a lifetime killer.
Many results of research for the purpose of specifying a defect being a lifetime killer of SiC or reducing the lifetime killer have been reported.
Mr. Zhang has specified a defect being a lifetime killer among electrically active defects (referred to as recombination centers or carrier traps) in an as-grown SiC layer by using the deep level transient spectroscopy (DLTS) and the minority carrier transient spectroscopy (MCTS) (Non-Patent Document 1). According to Non-Patent Document 1, electron traps by intrinsic defects and positive hole traps by boron impurities at the Z1/Z2 and EH6/7 centers are measured. Since a density of Z1/Z2 traps or a density of EH6/7 traps particularly indicates an inverse correlation with a lifetime, it is suggested that the Z1/Z2 traps or the EH6/7 traps are lifetime killers.
Mr. Hiyoshi has proposed the model in which thermal oxidation of an as-grown SiC epitaxial layer disperses interstitial carbon atoms ejected into the SiC layer in the thermal oxidation process, and thus the interstitial carbon atoms fill carbon vacancies in the as-grown SiC epitaxial layer, indicating that the thermal oxidation reduces a density of Z1/Z2 traps or a density of EH6/7 traps (Non-Patent Document 2).
Mr. Tsuchida has proposed the method of making traps electrically inert by implanting ions in an SiC crystal layer to additionally introduce interstitial carbon atoms in a shallow surface layer. Furthermore, by heating the SiC crystals, the interstitial carbon atoms additionally introduced in the surface layer are dispersed to the deeper area while the interstitial carbon atoms are combined with the carbon vacancies in the SiC crystal layer (Patent Document 1).
Mr. Kawahara has researched traps generated in ion implantation by ion-implanting impurity (dopant) atoms such as aluminum, phosphorus, and nitrogen in a surface of an SiC layer and also by performing DLTS evaluation on an element structure electrically activated by high-temperature annealing (Non-Patent Document 3). Non-Patent Document 3 discloses that particularly Z1/Z2 traps or EH6/7 traps are generated at a high concentration by the ion implantation and the traps are decreasingly distributed from the surface of the SiC layer toward the deeper area.
It has been proposed that dopant atoms such as aluminum and boron are ion-implanted in a surface of an SiC layer, and carbon atoms are simultaneously ion-implanted therein when an element structure is formed by electrically activating impurities in the surface layer through annealing in the process of manufacturing an SiC device (Patent Document 2). According to Patent Document 2, by ion-implanting carbon with boron in the surface of the SiC layer and introducing the surplus interstitial carbon atoms, the introduced surplus interstitial carbon atoms occupy carbon vacancies first, and boron is selectively introduced to silicon vacancies instead of the carbon vacancies at the time of annealing for electrically activating the impurities. As a result, it is indicated that a rate of boron that is electrically active is increased (a rate of activated boron is improved) more than the case of ion-implanting boron alone.